Single U- and Z-type parallel-channel configurations for gas distributor plates in planar fuel cells reduce the pressure drop but give rise to the problem of severe flow maldistribution wherein some of the channels may be starved of the reactants. In this paper, previous analytical solutions obtained for single U- and Z-type flow configurations are extended to multiple U- and multiple Z-type flow configurations of interest to fuel cell applications. Algorithms to calculate flow distribution and pressure drop in multiple U- and Z-type flow configurations are developed. The results are validated by comparison with those obtained from three-dimensional computational fluid dynamics (CFD) simulations. It is found that there is a significant improvement in the flow distribution in some configurations without paying for extra pressure drop. The possibility of unmatched distribution on the cathode and the anodes sides is also highlighted. Careful design of the flow configuration is therefore necessary for optimum performance. © 2005 Elsevier B.V. All rights reserved.

Sreenivas Jayanti

Department Of Chemical Engineering

Abhijit P. Deshpande

S. Maharudrayya

Algorithms

Computational fluid dynamics

Ion Exchange Membranes

Laminar flow

Pressure drop

Solutions

Flow distribution

PARALLEL CHANNELS

Proton-exchange membrane fuel cell

Fuel cells

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Journal of Power Sources

Mitigation of 'hot spots' in microelectromechanical systems (MEMS) employing in situ microchannel systems requires a comprehensive picture of the maldistribution of the working fluid and uniformity of cooling within the same. In this paper, detailed simulations employing parallel microchannel systems with specialized manifold-channel configurations, i.e., U, I, and Z, have been performed. Eulerian-Lagrangian discrete phase model (DPM) and effective property model with water and alumina-water nanofluid as working fluids have been employed. The distributions of the dispersed particulate phase and continuous phase have been observed to be, in general, different from the flow distribution, and this has been found to be strongly dependent on the flow configuration. Accordingly, detailed discussions on the mechanisms governing such particle distribution patterns have been proposed. Particle maldistribution has been conclusively shown to be influenced by various migration and diffusive phenomena, such as Stokesian drag, Brownian motion, thermophoretic drift, and so on. To understand the uniformity of cooling within the device, which is of importance in real-time scenario, an appropriate figure of merit has been proposed. It has been observed that uniformity of cooling improved using nanofluid as working fluid as well as enhanced relative cooling in hot zones, providing evidence of the 'smart' nature of such dispersions. To further quantify this smart effect, real-time mimicking hot-spot scenarios have been computationally probed with nanofluid as the coolant. A silicon-based microchip emitting nonuniform heat flux (gathered from real-time monitoring of an Intel Core i7-4770 3.40-GHz quad-core processor) under various processor load conditions has been studied, and the evidence of enhanced cooling of hot spots has been obtained from DPM analysis. This paper sheds insight on-the behavior of nonhomogeneous dispersions in complex flow domains and the caliber of nanofluids in cooling MEMS more uniformly and 'smarter' than base fluids. © 2011-2012 IEEE.

Fulltext

IEEE Transactions on Components, Packaging and Manufacturing Technology

Thermally 'Smart' characteristics of nanofluids in parallel microchannel systems to mitigate hot spots in MEMS

This paper brings out the phenomenon of flow maldistribution in parallel microchannel systems, which is supposed to have an adverse effect on hot spot formation and temperature distribution in microelectronic devices. An extensive experimental study is carried out where in the parameters affecting the flow maldistribution such as number of channel, area of cross section of the manifold, channel hydraulic diameter, and Reynolds number are varied to study their effect on the pressure drop across the parallel channels designed for liquid cooling of a CPU. It is observed that the flow distribution among the channels improves significantly with a decrease in the channel hydraulic diameter due to higher pressure drop offered by each individual channels simultaneously. This results in a considerable reduction in both the peak temperature and the average temperature of the device with decreasing channel diameters. It is also inferred that the flow maldistribution is relatively invariant with Reynolds number for the microchannel system, which is not the case for the macrochannels. Flow maldistribution is found to increase with increase in number of channels and with a decrease in the manifold area relative to the channel area. The 'I' type flow configuration is found to have the least maldistribution while the 'U' type shows the maximum and Z type falls in between. A simple force analysis of the governing equation in the manifold of the parallel microchannel system reveals a strong dominance of the frictional force over the inertial force and both the forces contribute to the uniform flow distribution at smaller hydraulic diameters, where the 1-D theoretical models failed to achieve a concurrence with the present experimental results. Also the present 3-D numerical simulations give a satisfactory agreement with the experimental results projecting it as an effective tool in the design and analysis of microchannel cooling system. The potential zones of hot spots are identified as low fluid velocity zones or low pressure drop zones among the channels resulting from flow maldistribution. © 2011-2012 IEEE.

Investigation on flow maldistribution in parallel microchannel systems for integrated microelectronic device cooling

Polymer Electrolyte Membrane (PEM) fuel cell is a promising energy conversion device with applications involving rapid start-up and low operating temperature. Proper cooling of PEM fuel cell stack is an essential requirement in ensuring its durability for which separate cooling channels between each cell are often used. This study involves a detailed three-dimensional numerical investigation on cooling channel designs based on traditional serpentine and spiral designs. Four new designs - divided serpentine, divided spiral, distributed serpentine and distributed spiral are proposed to predict the fluid flow and thermal characteristics. An in-house code based on Streamline upwind/Petrov Galerkin finite element method is used to solve the three-dimensional governing equations. Simulations are carried out for Reynolds number ranging from 415 to 1247. Results indicate that the novel designs have better performance compared to serpentine in terms of uniformity in temperature distribution at all Re. The merits and demerits of all the designs in terms of maximum and average temperature on the cooling plate is also discussed. The pressure drop required to drive the flow is higher in the spiral and new designs compared to serpentine due to the presence of complex turns. © 2014 Elsevier Ltd. All rights reserved.

Applied Thermal Engineering

Numerical studies on thermal performance of novel cooling plate designs in polymer electrolyte membrane fuel cell stacks

The presence of liquid water at the cathode of proton exchange membrane fuel cell hinders the reactant supply to the electrode and is known as electrode flooding. The flooding at the cathode due to the presence of two-phase flow of water is one of the major performance limiting conditions. A pseudo-two-dimensional analytical model is developed to predict the inception of two-phase flow along the length of the cathode channel. The diffusion of the water is considered to take place only across the gas diffusion layer (GDL). The current density corresponding to the inception of two-phase flow, called the threshold current density, is found to be a function of the channel length and height, GDL thickness, velocity, and relative humidity of the air at the inlet and cell temperature. Thus, for given design and operating conditions, the analytical model is capable of predicting the inception of two-phase flow, and therefore a flooding condition can be avoided in the first place. © 2010 American Society of Mechanical Engineers.

Journal of Fuel Cell Science and Technology

An analytical solution to predict the inception of two-phase flow in a proton exchange membrane fuel cell

This review brings out those aspects of the development of proton exchange membrane (PEM) fuel cells over the last two to three decades that are of interest to the heat and mass transfer community. Because the heat transport and mass transport in proton exchange membrane fuel cells are very important from the efficiency point of view, an emphasis is given here to these transports and their influence on operating cell parameters. The works are classified as models with either isothermal or non-isothermal conditions of various assumed dimensionality and with either single-phase or two-phase flow. Along with modeling, a few experimental studies available are also reported here. Researchers in the area of PEM fuel cells are involved in activities such as development of new and low-cost materials, modeling the relevant physical processes, and electrochemical experimentation. These collective efforts may lead to making this technology viable to meet world needs for clean and cheap energy. This review brings out the fact that computational fluid dynamics (CFD) has become an inevitable tool in fuel cell analysis, as the detailed interactions between the flow structure geometry, fluid dynamics, multiphase flow, heat transfer, mass transfer, and electrochemical reaction can be modeled simultaneously, given the present state of the art in CFD. Through the predictive capability of CFD, it will be possible for fuel cell designers to better optimize the design and operating parameters of fuel cells before testing them in laboratory.

Heat Transfer Engineering

Heat and mass transport in proton exchange membrane fuel cells - A review

Parallel-channel configurations for gas-distributor plates of planar fuel cells reduce the pressure drop, but give rise to the problem of severe flow maldistribution wherein some of the channels may be starved of the reactants. This study presents an analysis of the flow distribution through parallel-channel configurations. One-dimensional models based on mass and momentum balance equations in the inlet and exhaust gas headers are developed for Z- and U-type parallel-channel configurations. The resulting coupled ordinary differential equations are solved analytically to obtain closed-form solutions for the flow distribution in the individual channels and for the pressure drop over the entire distributor plate. The models have been validated by comparing the results with those obtained from three-dimensional computational fluid dynamics (CFD) simulations. Application of the models to typical fuel-cell distributor plates shows that severe maldistribution of flow may arise in certain cases and that this can be avoided by careful choice of the dimensions of the headers and the channels. © 2004 Elsevier B.V. All rights reserved.

Flow distribution and pressure drop in parallel-channel configurations of planar fuel cells

The pressure losses in the flow distributor plate of the fuel cell depend on the Reynolds number and geometric parameters of the small flow channels. Very little information has been published on the loss coefficients for laminar flow. This study reports a numerical simulation of laminar flow though single sharp and curved bends, 180° bends and serpentine channels of typical fuel cell configurations. The effect of the geometric parameters and Reynolds number on the flow pattern and the pressure loss characteristics is investigated. A three-regime correlation is developed for the excess bend loss coefficient as a function of Reynolds number, aspect ratios, curvature ratios and spacer lengths between the channels. These have been applied to calculate the pressure drop in typical proton-exchange membrane fuel cell configurations to bring out the interplay among the important geometric parameters. © 2004 Elsevier B.V. All rights reserved.

Journal	Journal of Power Sources
ISSN	03787753
Open Access	No